Heat tolerance limits of Mediterranean songbirds and their current and future vulnerabilities to temperature extremes

Abstract

Songbirds are one of the groups most vulnerable to extreme heat events. Although several recent studies have assessed their physiological responses to heat, most of them have focused solely on arid-zone species. We investigated thermoregulatory responses to heat in eight small-sized songbirds occurring in the Mediterranean Basin, where heatwaves are becoming more frequent and intense. Specifically, we determined their heat tolerance limits (HTLs) and evaporative cooling efficiency, and evaluated their current and future vulnerabilities to heat in southwestern Iberia, a Mediterranean climate warming hotspot. To do this, we exposed birds to an increasing profile of air temperatures (Ta) and measured resting metabolic rate (RMR), evaporative water loss (EWL), evaporative cooling efficiency (the ratio between evaporative heat loss and metabolic heat production) and body temperature (Tb). HTL ranged between 40 and 46°C across species, and all species showed rapid increases in RMR, EWL and Tb in response to increasing Ta. However, only the crested lark (Galerida cristata) achieved an evaporative cooling efficiency greater than 1. The studied songbirds currently experience summer Ta maxima that surpass the upper critical temperatures of their thermoneutral zone and even their HTL. Our estimates indicate that five of the eight species will experience moderate risk of lethal dehydration by the end of the century. We argue that the limited heat tolerance and evaporative cooling efficiency of small-sized Mediterranean songbirds make them particularly vulnerable to heatwaves, which will be exacerbated under future climate change scenarios.

Keywords: Climate warming; Heat stress; Heatwaves; Passerines; Thermal physiology; Thermoregulatory traits.

Fig. 1.

Resting metabolic rate (RMR) as a function of air temperature (T_a) in eight Mediterranean resident songbirds. (A) Serin, (B) goldfinch, (C) great tit, (D) chaffinch, (E) greenfinch, (F) house sparrow, (G) Spanish sparrow and (H) crested lark. RMR was regressed against T_a above the upper critical temperature (T_uc; see Table 1), obtaining a significant relationship in seven of the eight species. In the case of crested lark, we did not find a clear T_uc. Illustrations are reproduced with the permission of Juan Varela.

Fig. 2.

Evaporative water loss (EWL) as a function of air temperature (T_a) in eight Mediterranean resident songbirds. (A) Serin, (B) goldfinch, (C) great tit, (D) chaffinch, (E) greenfinch, (F) house sparrow, (G) Spanish sparrow and (H) crested lark. EWL was regressed against T_a above the upper critical temperature for this variable (see Table 1), obtaining a significant relationship in all species. Illustrations are reproduced with the permission of Juan Varela.

Fig. 3.

Evaporative cooling efficiency (EHL/MHP) as a function of air temperature (T_a) in eight Mediterranean resident songbirds. (A) Serin, (B) goldfinch, (C) great tit, (D) chaffinch, (E) greenfinch, (F) house sparrow, (G) Spanish sparrow and (H) crested lark. EHL/MHP was regressed against T_a above the upper critical temperature for this variable (see Table 1), obtaining a significant relationship in all species. Illustrations are reproduced with the permission of Juan Varela.

Fig. 4.

Body temperature (T_b) as a function of air temperature (T_a) in eight Mediterranean resident songbirds. (A) Serin, (B) goldfinch, (C) great tit, (D) chaffinch, (E) greenfinch, (F) house sparrow, (G) Spanish sparrow and (H) crested lark. T_b was regressed against T_a above the upper critical temperature for this variable (see Table 1), showing significant relationships in seven of the eight species. In the case of crested lark, we could not detect an inflection point for T_b. Illustrations are reproduced with the permission of Juan Varela.

Fig. 5.

Mean number of days per summer above the T_uc (see Table 1) experienced by the studied songbird species across Extremadura under current (2006–2021) and future (2070–2100) projected climate change scenarios (RCP4.5 and RCP8.5, respectively). (A) Serin, (B) goldfinch, (C) great tit, (D) chaffinch, (E) greenfinch, (F) house sparrow and (G) Spanish sparrow. Crested lark could not be mapped as we could not find a clear T_uc for this species. Illustrations are reproduced with the permission of Juan Varela.

Fig. 6.

Mean number of days per summer above HTL (see Table 1) for the great tit Parus major along Extremadura under current (2006–2021) and future (2070–2100) climate warming scenarios (RCP4.5 and RCP8.5, respectively). Great tit illustration is reproduced with permission from Juan Varela.

Fig. 7.

Current (2006–2021) and future (2070–2100) times to lethal dehydration (h) during an extremely hot day under the RCP4.5 and RCP8.5 climate warming scenarios for each of the Mediterranean songbird species studied. Dotted red line indicates a severe risk of lethal dehydration (i.e. birds losing 15% of M_b through EWL in ≤3 h), while dotted blue line indicates a moderate risk of lethal dehydration (i.e. when cumulative EWL surpassed 15% of M_b in ≤5 h).

Fig. 8.

Evaporative cooling efficiency (EHL/MHP) as a function of the gradient between air temperature and body temperature (T_a–T_b) for each of the Mediterranean songbirds studied. (A) Serin, (B) goldfinch, (C) great tit, (D) chaffinch, (E) greenfinch, (F) house sparrow, (G) Spanish sparrow and (H) crested lark. EHL/MHP was regressed against T_a–T_b above the upper critical threshold for this variable (which varies from −7.75°C in the greenfinch to −5.42°C in the house sparrow). Dashed lines indicate the intersection at which EHL/MHP achieve values close to 1 (horizontal dashed line) when T_a–T_b is 0 (vertical dashed line). Illustrations are reproduced with the permission of Juan Varela.